preClear

This method is called before the implementation calls the instance
method ClearCallback.jdoPreClear() and before it clears the values
in the instance to their Java default values. This happens during
an application call to evict, and in afterCompletion for commit
with RetainValues false and rollback with RestoreValues false.

The method is called during any state transition to hollow.
Non-persistent, non-transactional fields should be cleared
in this method. Associations between this instance and others
in the runtime environment should be cleared.

This method is not modified by the enhancer, so access to fields
is not mediated.